The present invention relates to apparatuses and methods for measuring three important physiological systems, circulation, breathing, and body water compartments. More specifically, the present invention relates to apparatuses and methods for a continuous simultaneous/synchronized and noninvasive measurement of heart pump function, elastic properties of the vascular system, systemic vascular resistance, breathing capacity, and body water compartments through the combined use of bioimpedance and continuous blood pressure measurement techniques and, optionally, spirometry and/or plethysmography.
The monitoring of the cardiovascular system and breathing is of major importance in the treatment and follow-up of critically ill patients, in the diagnosis of various disorders in the blood circulation and in the assessment and follow-up of medication of patients suffering from hypertension or cardiac malfunction. Typical parameters that reflect the status of cardiovascular and respiratory systems and are to be monitored are stroke volume, breathing related changes of the stroke volume, cardiac output, heart rate, arterial compliance, peripheral vascular resistance, breathing rate, breathing amplitude (tidal volume), and blood pressure. The monitoring of the cardiovascular parameters may also find use in scientific research and sports medicine. Cardiovascular system is inherently a multivariate closed-loop system. Main parameters of the system have a close mutual interrelation mediated by various mechanical and neural mechanisms. The autonomous nervous system controls different parts of this system by a continuous neural modulation causing small variations of the variables around their mean values. The breathing as an external stimulator modulates the hemodynamic parameters significantly. The relationship and interaction between the circulation and breathing is very tight, which emphasizes the need of simultaneous estimation of these systems in clinical work.
Body fluids play an important role in body homeostasis. The extracellular water retention is common in renal insufficiency and is also noted in cardiac insufficiency. Exaggerated dehydration may occur as a side-effect in treatment with diuretics, for example in patients with hypertension. Thus the simultaneous evaluation of the body water compartments would add essentially to a comprehensive and accurate evaluation of a patient's status.
At present the functional state of cardiovascular system and its coherence with respiration, vascular properties, and body fluids have been studied with separate measuring methods and devices at different stages. The combination of separate methods does not offer a reliable basis for the overall estimation of the patient's status, since the correlation of the results is inadequate and far from optimum. An establishment of the closed-loop interactions between different variables and a quantitative determination of cardiovascular parameters are not possible.
Only invasive methods have been considered reliable to measure the most important parameter of the heart pump function, the stroke volume, and its derivative, cardiac output (the blood volume pumped by the heart in one minute). The use of invasive methods in cardiac output measurement is expensive and is mostly restricted to critically ill patients. These methods involve a complication risk to the patient, e.g., catheter sepsis, bleeding, pneumothorax, and even death. There are nevertheless other patient groups who require hemodynamic evaluation, such as the patients with chronic heart failure, syncope, arterial hypertension, hemodialysis, etc.
Several approaches have been made to find an alternative to the invasive methods. An ideal method for the measurement of the cardiac output would be noninvasive and continuous. The impedance cardiography has both these features and has been an object of intensive research during the last 30 years. Noninvasive thoracic impedance methods for the measurement of the stroke volume originally described by Kubicek, W. G. et al. [Aerospace Med. 37 (1966) 1208-1212] have been evaluated extensively. However, the level of agreement with the invasive methods has varied widely indicating the imprecision of the thoracic methods [Fuller, H. D., Clin. Invest. Med. 15 (1992) 103-112; Mehisen, J. et al., Clin. Physiol. 11 (1991) 579-588; Pickett, B. R. and Buell, J. C., Am. J. Cardiol. 69 (1992) 1354-1358; Yakimets, J. and Jensen, L., Heart Lung 24 (1995) 194-206; Atallah, M. M. and Demain, A. T., J. Clin. Anaesthesia 7 (1995) 182-185]. Furthermore, the thoracic impedance methods provide information only about a limited number of cardiovascular parameters, no information can be obtained, eg., about the whole body water compartments or different peripheral pulsatile parameters.
Based on the use of a thoracic bioimpedance measurement, U.S. Pat. No. 4,807,638 discloses a method and a device for a noninvasive continuous monitoring of mean arterial blood pressure and certain cardiac parameters, namely cardiac index, left cardiac work index and systemic vascular resistance index. However, it is well known in the art that a blood pressure measurement based on impedance is not reliable.
In the 1970s Tishchenko developed an alternative impedance method, the whole-body impedance cardiography known also as integral rheography and integral impedance plethysmography [Tishchenko, M. I., Sechenov Physiol. J. 59 (1973) 1216-1224; Tishchenko, M. I., et al., Kardiologiia 13(11) (1973) 54-62]. The whole-body impedance cardiography differs from thoracic impedance methods in its placement of electrodes, the lower frequency of the alternating current used, and the stroke volume equation. Even though the whole-body impedance cardiography has shown excellent agreement with the thermodilution, which currently is the most widely used invasive method for the measurement of cardiac output [T. Koobi et al., Medical & Biological Engineering & Computing 34, Suppl. 1, Part 2, (1996) 163-164 (Proceedings of the 1st International Conference on Bioelectromagnetism. Jun. 9-13, 1996, Tampere, Finland)], it alone cannot provide sufficient data to allow a reliable overall assessment of cardiorespiratory condition of the patient.
The need for a simple and noninvasive diagnostic apparatus and methods for simultaneous measurement of cardiorespiratory status is evident.
The object of the present invention is to provide an apparatus and a method for a continuous, simultaneous/synchronized noninvasive measurement of cardiorespiratory and related parameters.
Another object of the present invention is to provide a apparatus and a method for a continuous, simultaneous/synchronized noninvasive measurement of cardiorespiratory and related parameters producing reliable, quantitative data for the establishment of the patients status on a real-time basis.
Yet another object of the present invention is to provide an apparatus and a method for a continuous, simultaneous/synchronized noninvasive measurement of cardiorespiratory and related parameters providing common basis for a reliable determination of the patient's status.
Yet another object of the present invention is to provide an apparatus and a method for a continuous, simultaneous/synchronized noninvasive measurement of cardiorespiratory and related parameters enabling a simple manipulation of the patient at a time without causing stress to the patient.
Yet another object of the present invention is to provide an apparatus and a method for a continuous, simultaneous/synchronized noninvasive measurement of cardiorespiratory and related parameters based on a simultaneous/synchronized noninvasive measurement of bioimpedance in the whole body and body segments on heartbeat-to-heartbeat basis and the blood pressure and, optionally, spirometry and/or plethysmography.